- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] Re: best Linux for GHC?
by Luis F. Araujo 13 Nov '06

13 Nov '06

Alex Queiroz wrote: >> Hallo, >> >> On 11/13/06, Mark T.B. Carroll <Mark.Carroll(a)aetion.com> wrote: >> >>> How up to date will Debian unstable's GHC be kept, though? It seems >>> pretty good at the moment, but there have been times when we've had to >>> install from source instead of via Debian earlier this year so that we >>> had GHC features and bugfixes in place - the Debian version has got >>> rather out of date at times with respect to stuff we needed. >>> >>> >> No OpenGL neither Gtk2hs in Debian Sid yet. :-( >> I'd recommend Gentoo for using these kind of packages. We even have darcs repository versions of Gtk2Hs in our gentoo haskell overlay. Regards,

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Section Syntax Errors
by Aditya Siram 13 Nov '06

13 Nov '06

Hi all, I am learning about sections and currying from SOE. There are three functions below. The first is from SOE and the other two are my own practice functions. Why does the third function fail? --Test for negative numbers (from SOE pg 109). This works posInts :: [Integer] -> [Bool] posInts = map (> 0) --Add 1 to a list of numbers. This works. addOne :: [Integer] -> [Integer] addOne = map (+ 1) --Subtract 1 from a list of numbers. This does NOT work. --Fails in Hugs with: -- Instance of Num (Integer -> Integer) required for definition of testSections -- I tried : subOne :: Num a => [a] -> [a] -- and : subOne :: [Float] -> [Float] --in case the problem is with negative numbers. They both fail. --Without any type signature I get: -- Unresolved top level overloading subOne :: [Integer] -> [Integer] subOne = map (- 1) Thanks... Deech _________________________________________________________________ Add a Yahoo! contact to Windows Live Messenger for a chance to win a free trip! http://www.imagine-windowslive.com/minisites/yahoo/default.aspx?locale=en-u…

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Great language shootout: reloaded
by tpledger＠ihug.co.nz 13 Nov '06

13 Nov '06

Donald Bruce Stewart wrote: [...] > While we're here we should fix: > chameneos > And anything else you want to take a > look at. > > A community page has been set up to > which you can submit improved entries: > http://www.haskell.org/haskellwiki/Great_language_shootout [...] Well, then! I've put a new chameneos solution up on the wiki, and will wait the recommended couple of days for Community Feedback. - Tom

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ByteString FFI
by Donn Cave 13 Nov '06

13 Nov '06

How do people like to set up their foreign I/O functions to return ByteStrings? I was a little stumped over this yesterday evening, while trying to write ` recv :: Socket -> Int -> Int -> ByteString ' Doc says `Byte vectors are encoded as strict Word8 arrays of bytes, held in a ForeignPtr, and can be passed between C and Haskell with little effort.' Which sounds perfect - I'm always up for `little effort'! CString doesn't seem like the right thing for socket results, because the data shouldn't be NUL-terminated, and I might want to realloc when the returned data doesn't fill the buffer. I don't see any other Ptr-related function or constructor in the documentation - am I missing something there? Thanks, Donn Cave, donn(a)drizzle.com

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GHCi 6.6 tab-completion of source file paths not working.
by Daniel McAllansmith 13 Nov '06

13 Nov '06

Hi. I've just installed GHC 6.6 on an amd64 running a gentoo linux distribution. With GHCi from 6.4.2 I could run "ghci" then do Prelude> :l Foo/Bar.hs by hitting tab after Foo to complete the path to Bar.hs This no longer works, hitting tab only shows what's in the pwd. If I run "ghci Foo/Bar.hs" then I can :r fine and I can do a :l Foo.Bar using tab-completion to complete the module name. Tab completion of in-scope functions looks to be working fine. Has anybody else experienced this problem? Thanks Daniel

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A type class puzzle
by Yitzchak Gale 13 Nov '06

13 Nov '06

Consider the following sequence of functions that replace a single element in an n-dimensional list: replace0 :: a -> a -> a replace0 = const replace1 :: Int -> a -> [a] -> [a] replace1 i0 x xs | null t = h | otherwise = h ++ (replace0 x (head t) : tail t) where (h, t) = splitAt i0 xs replace2 :: Int -> Int -> a -> [[a]] -> [[a]] replace2 i0 i1 x xs | null t = h | otherwise = h ++ (replace1 i1 x (head t) : tail t) where (h, t) = splitAt i0 xs etc. Generalize this using type classes.

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(twice head) && (twice tail)
by Greg Buchholz 13 Nov '06

13 Nov '06

Over on comp.lang.functional ( http://xrl.us/s6kv ), Toby Kelsey is wondering about writing a function "twice" that applies a function to an argument two times... > twice' :: (a -> a) -> a -> a > twice' f x = f (f x) ...that works for things like "twice succ 0" and "twice tail [1,2,3]", but the type signature means you can't try something like "twice head [[1]]". You can get the "twice head" case to work with a function like... > twice'' :: (forall a. (m a) -> a) -> (m (m b)) -> b > twice'' f x = f (f x) ...and if you want something like "twice (flip (:) [])", you'd use... > twice''' :: (forall a. a -> (m a)) -> b -> (m (m b)) > twice''' f x = f (f x) ...it seems like those type signatures have at least a passing resemblance, so I was wondering if you could use type classes to combine these functions. I tried something like... > class Twice a b c | a b -> c where > twice :: a -> b -> c > > instance Twice (a->a) a a where > twice f x = f (f x) > > instance Twice (forall a. (m a) -> a) (m (m b)) b where > twice f x = f (f x) ...but with ghc-6.6 it chokes with... Illegal polymorphic or qualified type: forall a. m a -> a In the instance declaration for `Twice (forall a. (m a) -> a) (m (m b)) b' ...I can't say I'm surprised, but I was wondering if anyone else has thoughts on how this limitation might be worked around. Curious, Greg Buchholz

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Simple implementations of the lambda calculus
by Lennart Augustsson 12 Nov '06

12 Nov '06

Some time ago I made a little experiment and implemented an interpreter for the lambda-calculus in Haskell. The only reason was that I wanted to try different ways of doing it. So I did it using simple substitution (i.e., as the textbooks describe it), unique identifiers, deBruijn indicies, and higher order abstract syntax. You can get the code from http://darcs.augustsson.net/Darcs/Lambda/ or just check out small paper at http://darcs.augustsson.net/Darcs/ Lambda/top.pdf -- Lennart

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Re: Livecoding music in Haskell
by Rohan Drape 11 Nov '06

11 Nov '06

On Tue Nov 7 16:32:11 EST 2006, alex wrote: > The way I see it there are two big issues - the first is drift and the > second is latency. As hinted at when Alex's work was discussed last November: "OSC messages can be timestamped, and SuperCollider has a sample accurate scheduling queue, so language timing jitter can easily be worked around." http://www.haskell.org/pipermail/haskell-cafe/2005-November/012483.html Currently Sound.SC3 has a procedure 'at' that can be used for scheduling. This procedure doesn't really belong in Sound.SC3, and ought probably be taken out. Still, with the current darcs repository the following makes a ping every second, on the second, sample accurately, for half a minute. If you run the binary twice the pings will be twice the amplitude, no phase errors - fingers crossed. import Sound.SC3 import Control.Concurrent (forkIO) ping f a = out 0 (sinOsc AR f 0 * e) where c = EnvNum (-4.0) e = envGen KR 1 a 0 1 removeSynth (envPerc 0.1 0.6 1 [c,c]) latency = 0.01 bundle t m = OscB (t + latency) m pinger = do now <- utc at (fromIntegral (ceiling now)) f where f t = do fd <- sc send' fd (bundle t [s_new "ping" (-1) AddToTail 1]) putStrLn "Sending ping" return 1.0 main = do fd <- sc putStrLn "Sending Ping Instrument" sync' fd (d_recv' "ping" (ping 440 0.1)) putStrLn "Resetting scsynth" reset fd putStrLn "Starting schedule thread" forkIO pinger putStrLn "Delaying main thread" pause 30 putStrLn "End of delay, exiting" The above assumes that scsynth is running on the local host at the standard port, 57110, and that the GHC runtime scheduler jitter plus localhost network latency for this task is below 0.01 seconds, which is true on my otherwise idle X31 at 600MHz - this is not at all bad, I am impressed in any case - setting latency to zero gives reports from scsynth of: > late 0.008414722 > late 0.006882722 > late 0.005348722 > late 0.003815721 > late 0.002282721 > late 0.000748721 Tacked on below, for interested readers, are some notes on a related scheme scheduler, the notes were written in response to a related query about scheme & scsynth some time ago. The relation to the haskell above is pretty straightforward, the haskell 'at' discards the notion of a mutable schedule - with cheap concurrency such a thing is of arguable use - and the haskell 'at' ought to allow the event generator to return Nothing to stop scheduling. Regards, Rohan ++ Simple sample accurate scheduling from runtimes with moderate scheduling jitter is straightforward using SuperCollider. One simple model is: (at Q TIME (lambda (t f) (EVENT t) (f DELTA))) at = the scheduler interface Q = a <schedule> value TIME = a UTC timestamp t = the scheduled UTC time (ie. TIME or subsequent delta), regardless of when the procedure actually runs f = a rescheduling function that in effect does (at Q (+ t DELTA) *SELF*) EVENT = the action, usually constructs an osc bundle and sends it to scsynth DELTA = the delta time to reschedule to, to not re-schedule just don't call f The EVENT sends a bundle to scsynth and adds latency as required so that the scheduled bundle arrives ahead of the timestamp, the actual sample-accurate scheduling is handled by a queue at scsynth. The example below will schedule a ping at each whole second, and the scheduling will be sample accurate so long as the scheme runtime jitter is less than 0.1 seconds minus the network latency to get a UDP packet to the scsynth address. Here (utc) gets the current time, (-> s p) sends an OSC packet p to the server s, (/s_new ...) makes a /s_new OSC message, & (bundle t m) makes an OSC packet converting the UTC timestamp to NTP. (define s (open-udp* "127.0.0.1" 57110)) (define Q (make-schedule*)) (define L 0.1) (define (ship t m) (-> s (bundle (+ t L) m))) (at Q (ceiling (utc)) (lambda (t f) (ship t (/s_new "ping" -1 1 1)) (f 1.0))) Obviously to schedule just one ping in five seconds time: (at Q (+ (utc) 5) (lambda (t _) (ship t (/s_new "ping" -1 1 1))))

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Great language shootout: reloaded
by Isaac Gouy 11 Nov '06

11 Nov '06

Donald Bruce Stewart wrote: -snip- > I agree. Breaking the rules was mainly the reason for the drop. Entries > like chameneos and fasta. Also, the other language teams kept improving > things. Yes, I missed that opportunity for listing things in threes ;-) Over the year improved programs were contributed for other languages. In contrast to the last focussed effort by the Haskell community, many of the other programs are contributed by individuals working alone, over many weeks. > > Other language (perl, iirc) were affected far worse by the gzipping. > gzip is an interesting measurement, and it doesn't hurt Haskell too much > either way -- short Haskell programs stay short when compressed. > > As a result, rewriting verbose entries to ByteString will probably be > much more useful :) > > Btw, Isaac, are we going to have any new parallelism benchmarks? I'd > love to try out the SMP runtime ;) If we were to have a 'which is the silliest comparison on the computer language shootout' competition, I think chameneos and cheap-concurrency would be joint first - there are so many really really different approaches. Ideas for unicode text processing would be more welcome. ____________________________________________________________________________________ Cheap talk? Check out Yahoo! Messenger's low PC-to-Phone call rates. http://voice.yahoo.com

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